1. Field of the Invention
The present invention generally relates to a filter circuit to be provided in a radio communication module for microwave and millimeter wave communications, and more particularly, to a filter circuit adjustable to a predetermined bandpass frequency characteristic.
2. Description of the Related Art
Along with the evolution of the information communication technology, the radio communication module is used in various devices and systems such as mobile communication devices, ISDN (integrated service digital network) or computer devices to enable fast communications of data and information, and a small and lightweight design of them, a higher integration or higher multiplication of their functions. In the applications of communications, such as a radio LAN (local area network), in which a frequency in the microwave and millimeter wave bands is used as a carrier frequency, the radio communication module can hardly meet the above-mentioned required specifications such as the small and lightweight design, higher integration and multiplication of functions by any lump parameter design-based circuit using a chip part such as a capacitor or coil as a low-pass filter, high-pass filter, bandpass filter or coupler. To meet such required specifications, the filter circuit has normally be constructed by the method of distributed parameter design using a micro strip line, strip line or the like.
Referring now to FIG. 1, there is illustrated in the formed of a plan view a conventional bandpass filter (BPF) constructed by the method of distributed parameter design. The BPF is generally indicated by a reference 100. As shown in FIG. 1, the BPF 100 includes a dielectric substrate 101 and has a plurality of resonator conductive patterns 102a to 102e formed like a cascade on the main side (along a micro strip line) of the dielectric substrate 101. The BPF 100 is supplied at one outer conductive pattern 102a thereof with a radio frequency signal, selects a predetermined carrier frequency band at the inner conductive patterns 102b to 102d, and outputs the frequency band at the other outer conductive pattern 102e. The conductive patterns 102 except for the middle one 102c are connected to each other at the opposite sides of the substrate 101. The substrate 101 has a ground pattern (not shown) formed over the rear side thereof.
In the BPF 100, two adjacent ones of the conductive patterns 102a to 102e are formed on the main side of the dielectric substrate 101 to overlap each other in a range of a quarter (¼) of a bandpass wavelength λ. Since the conductive patterns 102 are formed on the substrate 101 having a high dielectric constant, the BPF 100 can be designed small by reducing the length of each conductive pattern 102 owing to the effect of wavelength reduction of the micro strip line. The wavelength reduction can be attained at a rate of λ0/√εω (where λ0 is a wavelength in vacuum, and εω is an effective specific inductive capacity; dielectric constant depending upon an electromagnetic field distribution in air and dielectric material) on the surface of the substrate 101, and at a rate of λ0/√εγ (εγ is a specific inductive capacity of the substrate) inside the substrate 101. Also, since the conductive patterns 102 can be formed on the main side of the substrate 101 as in the ordinary wiring board forming process by printing or lithography, the BPF 100 can be formed simultaneously with a circuit pattern or the like.
However, since the conductive patterns 102a to 102e are formed with the two adjacent ones laid to overlap each other in the range of λ/4, the substrate 101 has to be wide enough for such a layout of the conductive patterns 102. Thus the BPF 100 depends in size upon the substrate 101 and can be designed to have a limited small size.
FIG. 2(A) to FIG. 2(C) and FIG. 3 show together a bandpass filter (BPF) of a conventional tri-plate structure. This BPF is generally indicated with a reference 110. As shown, the BPF 110 has a so-called tri-plate structure in which resonator conductive patterns 113 and 114 are formed between a pair of dielectric substrates 111 and 112 joined to each other. The dielectric substrates 111 and 112 have ground patterns 115 and 116 formed over the outer surfaces, respectively, thereof. Also, the dielectric substrates 111 and 112 have multiple vias 117 formed along the peripheries, respectively, thereof. The front-and rear-side ground patterns 115 and 116 are electrically connected to each other to shield the internal circuit.
The resonator conductive patterns 113 and 114 have a length l nearly equal to a quarter (¼) of the bandpass wavelength λ. They are connected at one end thereof to the ground patterns 115 and 116 and extend in parallel to each other with their other ends being open-circuited. Further, the resonator conductive patterns 113 and 114 have input and output patterns 118 and 119, respectively, formed thereon to project laterally like an arm. Thus, in the BPF 110, the dielectric substrates 111 and 112 and the resonator conductive patterns 113 and 114 are capacitively coupled like an equivalent circuit to provide a parallel resonant circuit, as shown in FIG. 3.
In the aforementioned BPF 110, the frequency characteristics such as passband characteristic, cut-off characteristic and the like depend upon the electromagnetic field distribution between the dielectric substrates 111 and 112 and resonator conductive patterns 113 and 114. In the BPF 110, the field strength varies depending upon the space p between the resonator conductive patterns 113 and 114 in the mode of odd excitation, and depending upon the space between the dielectric substrates 111 and 112 and resonator conductive patterns 113 and 114, that is, the thickness t of the dielectric substrates 111 and 112, in the mode of even excitation. Also, in the BPF 110, the field strength varies depending upon the width w of the resonator conductive patterns 113 and 114.
In the BPF 110, as the field strength varies in the modes of odd excitation and even excitation, the degree of coupling between the resonator conductive patterns 113 and 114 varies and thus the filter characteristic varies. To assure a predetermined filter characteristic, the dielectric substrates 111 and 112 and the resonator conductive patterns 113 and 114 in the BPF 110 are formed with a high precision.
If the manufacturing dimensional precision of each component of the BPF is not always constant, the BPF cannot show a desired filter characteristic in some cases. To avoid this, an adjustment of the BPF has to be done as an additional job by appropriately changing the position, area and the like of the resonator conductive patterns while checking their output characteristic using a measuring instrument, for example. However, the BPF 110 cannot easily be adjusted in such a manner since the resonator conductive patterns 113 and 114 are formed inside the dielectric substrates 111 and 112 as having been described above. Since the components of the BPF 110 have to be produced with a high precision, the BPF 110 cannot be produced with any improved efficiency and yield.